PHOENIX — For the first six months of this fiscal year, ending in March, Border Patrol apprehensions of undocumented migrants entering from Mexico climbed 13 percent compared with a year earlier.

The new data come as the debate over immigration reform and how much more to spend locking down the border intensifies in Washington. The Senate is expected to soon begin considering a sweeping bipartisan reform bill.

The increase in apprehensions generally signals an increase in attempted crossings. And the recent surge is coming almost entirely through the Rio Grande Valley in Texas, which is beginning to rival the Tucson Sector as the busiest — and deadliest — route for migrants from Mexico.

But while a 13 percent increase may sound dramatic, the number of apprehensions, 189,172, is still near historical lows. In the last 40 years, only the numbers for the first halves of fiscal 2011 and 2012 were lower.

Does this rise mean that, as the U.S. economy grinds toward recovery, we can expect a return to the massive migration levels of, say, 2000, when Border Patrol agents apprehended nearly 1.7 million migrants and couldn’t begin to count how many more got past them?

Independent of the far greater resources now — 651 miles of fencing, extensive surveillance and more than 18,000 Border Patrol agents deployed along the southwestern border — “there’s been a fundamental shift in migration patterns from Mexico,” Mittelstadt said.

“The demographics (in Mexico) have changed: Fertility rates have dropped, there are fewer people of young age who are the likeliest to migrate, the economy is improving, educational attainment is improving.”

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David FitzGerald, a sociologist at the Center for Comparative Immigration Studies at the University of California, San Diego, agreed.

“I do think the job market will see higher migration than now as the economy picks up,” said FitzGerald, who has been surveying migrants from central and western Mexico since 2005. But, he said, for a variety of reasons, “I think it’s highly unlikely we’ll reach the levels we saw a decade ago, no matter what happens with immigration reform.”

FitzGerald said increased border security has had some impact on migration, by channeling migrants into more remote and dangerous areas. In his most recent survey, in January, people who decided not to cross the border most often cited their fears of the natural hazards of crossing the desert, and of the dangers of crossing areas in northern Mexico controlled by violent drug cartels.

Last fiscal year was the second-deadliest on record for migrants crossing the border, with 463 bodies found by the Border Patrol or local agencies. Only 2005, with 492 deaths, was worse. The Tucson Sector recorded 177 deaths last fiscal year; the Rio Grande Valley Sector, 150.

The apprehension data also show other significant shifts. For more than a decade, the Tucson Sector has seen the highest number of apprehensions of any Border Patrol sector. But in March, it was surpassed for the first time by the Rio Grande Valley Sector, which lies at the southeastern end of Texas, stretching to the Gulf Coast.

In March, the Tucson Sector had 15,009 apprehensions, compared with 16,100 in the Rio Grande Valley Sector. The Tucson Sector’s March apprehensions fell 9 percent from a year earlier; the Rio Grande Valley’s rose 67 percent.

The Border Patrol confirmed that in southern Texas, agents are seeing increasing numbers of migrants from Central America, especially El Salvador, Guatemala and Honduras. Mittelstadt said that jibes with the patterns the Migration Policy Institute is seeing.

But the fact that migration is coming from Central America is another reason that the flows won’t reach the levels of a decade ago, she suggested, because El Salvador, Guatemala and Honduras have a combined population less than one-fifth that of Mexico.